Polymers of diethylene glycol bis(allyl carbonate), which is known commercially in its monomeric form as allyl diglycol carbonate and in either its monomeric or polymerized form as CR-39 (a tradename of the Chemical Division of the Pittsburgh Plate Glass Company), form a clear plastic having good optical and scratch-resistant properties and, as such, are used primarily in the production of optical lenses and of flat, transparent sheets. See, for example, Encyclopedia of Polymer Science and Technology, Vol. 1, pages 799-803, Interscience Publishers, New York, 1964.
Diethylene glycol bis(allyl carbonate) typically is polymerized with free-radical initiators to give cross-linked, thermoset materials. Since the monomer is relatively unreactive, about 3 to 5% of initiator based on the weight of monomer is typically required. Commonly used initiators are di-isopropyl peroxydicarbonate, known as IPP in the trade, and benzoyl peroxide.
For the manufacture of ophthalmic-quality thermoplastic lenses, diethylene glycol bis(allyl carbonate) is mixed with the initiator and cast in a mold having the appropriate curvature. The time of polymerization, or cure cycle, is a function of the rate at which the heat of polymerization can be removed. Typically the lens is cured by placing the charged mold either in an air oven or a water bath for up to twenty-four hours.
One of the problems of casting diethylene glycol bis(allyl carbonate) in such molds is shrinkage of the monomer during polymerization, a substantial portion of which polymerization occurs within the molds themselves. For example, diethylene glycol bis(allyl carbonate) can shrink as much as 14% in volume during polymerization. Such shrinkage imposes stresses on the casting formed in the mold due to adherence of the casting to the mold surface. These stresses cause the lens casting either to lose intimate contact with the mold surface and/or cause cracking of the lens castings or the mold itself. In sum, shrinkage causes changes in dimensions, internal stresses in the cast lenses, cracking and/or separation from the mold walls yielding surfaces of inadequate smoothness.
It is known that volume shrinkage during polymerization can be reduced to some degree by either of two methods: either a "syrup" can be formed by dissolving a previously prepared polymer in the monomer to be cast or the monomer can be prepolymerized to low conversion before charging it to the mold.
By the first known method, addition of a previously prepared polymer often results in a phase separation from the diethylene glycol bis(allyl carbonate) resin matrix such that the cured system is rendered opaque or transparent with striations and unsuitable as an ophthalmic lens.
By the second known method, conventional prepolymerization before charging into the molds has not successfully been used effectively to reduce subsequent shrinkage of diethylene glycol bis(allyl carbonate) during complete polymerization. Prepolymerization cannot substantially reduce shrinkage because low conversion is suggested. Furthermore, a prepolymerization step in conventional manner with high amount of initiator tends to overpolymerize the composition, producing a gel that excludes its further use.
It can thus be readily appreciated that provision of a method of prepolymerizing a monomer composition containing diethylene glycol bis(allyl carbonate) which confers on the art the advantage of reducing volume shrinkage during polymerization while casting in a glass mold and increases the yield of ophthalmic plastic lenses from glass molds, and eliminates the previously discussed problems, would be a highly desirable advance over the current state of technology.